The purpose of this report was to illustrate how thoracic gas compression (TGC) artifact, and differences in air density, may together conflate the inter-pretation of changes in the forced expiratory flows (FEFs) at high altitude (>2400 m). Twenty-four adults (10 women; 44 +/- 15 year) with normal base-line pulmonary function (>90% predicted) completed a 12-day sojourn at Mt. Kilimanjaro. Participants were assessed at Moshi (Day 0, 853 m) and at Bar-afu Camp (Day 9, 4837 m). Typical maximal expiratory flow-volume (MEFV) curves were obtained in accordance with ATS/ERS guidelines, and were either:(1) left unadjusted; (2) adjusted for TGC by constructing a “maximal perime-ter” MEFV curve; or (3) adjusted for both TGC and differences in air density between altitudes. Forced vital capacity (FVC) was lower at Barafu compared with Moshi camp (5.19 +/- 1.29 L vs. 5.40 +/- 1.45 L, P < 0.05). Unadjusted data indicated no difference in the mid-expiratory flows (FEF25–75%) between altitudes (Δ + 0.03 +/- 0.53 L sec+/-1; Δ + 1.2 +/- 11.9%). Conversely, TGC-adjusted data revealed that FEF25–75% was significantly improved by sojourn-ing at high altitude (Δ + 0.58 +/- 0.78 L sec􀀂1; Δ + 12.9 +/- 16.5%, P < 0.05). Finally, when data were adjusted for TGC and air density, FEFs were “less than expected” due to the lower air density at Barafu compared with Moshi camp (Δ–0.54 +/- 0.68 L sec+/-1; Δ–10.9 +/- 13.0%, P < 0.05), indicating a mild obstructive defect had developed on ascent to high altitude. These findings clearly demonstrate the influence that TGC artifact, and differences in air den-sity, bear on flow-volume data; consequently, it is imperative that future investigators adjust for, or at least acknowledge, these confounding factors when comparing FEFs between altitudes.